There are several common uses for mills which are used to pulverize solids. One example is the pulverization of coal to prepare it for firing in a furnace.
One type of mill, commonly known as a ball mill, as its name implies, uses large metal balls to pulverize the material. Ball mills are generally reliable but have several disadvantages. For any given quantity of coal or other material to be pulverized they are comparatively large and therefore require a rather large capital expenditure. Further, they consume much energy in their operation.
When a less expensive will is required, roller mills are often employed. Roller mills which are presently used to pulverize coal generally employ several rollers which bear down on a rotating table onto which the coal is fed to pulverize the coal. In some designs, the rollers have flat tread portions which tend to wear unevenly over a period of time. This results in a pulverized coal in which the particulate size is not uniform. Other designs have structure which urge the rollers down against the rotating table in such a way that when the coal between the table and roller exerts an upward force on a roller, the other rollers are jostled. The result is the creation of vibration during operation and a consequent excessive degree of wear and a loss of efficiency.
In other designs, the rollers begin to yaw after a period of operation because of wear in the parts which are designed to guide the rollers. This creates excessive rolling friction so that the rollers often skid over the table rather than roll over it. When this happens, the skidding roller is not pulverizing the coal properly, and consequently the efficiency of the mill quickly deteriorates.